Let's look at the numbers game behind the rhetoric at the national political conventions. Each party wants to assemble a coalition of voters just large enough to win. Democrats and Republicans alike are saying things that are shaped by political math.

Ron Brownstein of The National Journal has spoken with both campaigns about the racial breakdown of the vote. Four years ago, President Obama captured around 80 percent of the minority vote. His campaign hopes to do at least that again, and match the minority vote with just enough white voters to win. Mitt Romney, like most Republicans, expects to win the white vote. But because the minority vote has been growing, Romney has to win the white vote big, maybe with 60 percent or more.

You quote an unnamed Republican strategist who says, quote, "This is the last time anyone will try to do this." What is it that they're trying to do for the last time here?

RON BROWNSTEIN: I think what they're trying to do for the last time is build a national majority almost entirely on the backs of white votes in a country that is rapidly diversifying. You know, we're up to a point where 37 percent of the overall population and probably a little more than a quarter of the electorate is going to be non-white. And yet, in 2008, roughly 90 percent of John McCain's votes came from whites. In this year's primaries, over 90 percent of the cumulative votes in the Republican primary were cast by whites.

So essentially, you have a Republican Party that, at this point, is almost exclusively dependent on the votes of whites, in a country that is on the track toward becoming majority minority sometime in the next 30 years.

INSKEEP: So the Republicans having a much whiter coalition are at a greater disadvantage seemingly almost every election. Is that what you're saying?

BROWNSTEIN: Well, the share of whites they need to win grows every four years, unless they can reduce this preponderant Democratic advantage among minorities. You really can feel the paralysis right now in the Republican Party, because on the one hand, almost every serious thinker in the party understands this is not a tenable, long-term strategy, to allow Democrats to keep winning 80 percent of the growing minority population.

On the other hand, their existing coalition is resistant to many of the things you would want to reach out to that population. And they don't know how to square the need for a future coalition against the priorities of the existing coalition.

INSKEEP: What about their existing coalition is resistant to including more people of color?

BROWNSTEIN: Well, I think there are a couple things. I mean, first of all, they should have mechanisms and arguments that will allow them to appeal especially to the Hispanic and Asian communities. There are a lot of small business in both. The Hispanic community is culturally conservative on things like gay marriage. But they have not been able to get to first base because of the immigration issue.

Polls tell us that immigration is not the most important issue for Hispanics. They worry mostly about the economy and education and health care. But it is a threshold issue. It is a threshold issue in terms of showing respect for the community. And what you've got now is a Republican Party that is dependent on such historic margins - which they are going to see in this election - among white seniors and blue-collar whites who also tend to be the most resistant to things like comprehensive immigration reform or any kind of outreach.

They're struggling between, as I say, the coalition they have and the coalition they know they have to build.

INSKEEP: You know, we've looked at this from Romney's point of view, that he's doing so poorly among minorities that he needs a huge percentage of the white vote. Let me flip it around and ask about it from President Obama's point of view. Isn't this a president who did quite well for a Democrat among white voters in 2008? And what does it say that now he's struggling to get 40 percent if he can?

BROWNSTEIN: That is a very good point. Obama got 43 percent of white voters in '08, which is basically in the range of Democrats. But they simply have not been able to formulate their agenda, their vision of activist government in the way that can hold white support when they are implementing it. You know, in 2010, Democrats lost the highest share of the white vote they've ever lost in a midterm election.

Neither side, I think, is fundamentally advantaged by a politics that is this deeply racially polarized, and neither is the country.

INSKEEP: What are some other ways that these demographics are informing the messages that we're seeing on TV, the statements we're seeing online, or the mechanics on the ground?

BROWNSTEIN: Well, enormously on each side. First of all, I think what the Obama campaign really is doing, in some ways, is a 21st century version of a Walter Mondale interest group liberalism, because it's not just minorities. What they have done is they have looked at each sliver of their coalition and try to find an issue or issues that will activate and mobilize them. So if you look at the Obama coalition, what are they doing?

Well, for young people, they picked a big fight about student loan rates this summer. Also, I think, is gay marriage decision. Minorities, he's emphasized the DREAM Act for Hispanics. The administrative action on the DREAM Act changed his deportation policy. Earlier this year, college-educated white women, he picked a big fight over contraception and the access to contraception in the health care plan.

To me, the common thread in all of this is that if you look at the positions Obama has taken on these questions, he has been willing to accept greater conflict and greater risk of losing culturally conservative whites. Now, the flip side, the Republican message, I think, is centered on the idea of going back to a very '80s-style argument about who is on your side.

I thought the most important thing that's happened in the last few weeks is I feel that the Republican answer to the charge that Mitt Romney favors the rich over the middle class has emerged, and the answer is that Barack Obama favors the poor over the middle class. That is the message of the welfare ad. That is the message, I think, even more directly of the Medicare ad, where they say that Obama has taken $716 billion from Medicare for a program that's not for you.

The basic argument - as one Republican consultant said on TV the other night - is it's makers versus takers, and that is a very '70s and '80s kind of construct. Clinton - Bill Clinton tried very hard to kind of dispel that line of argument. But austerity, I think, has brought it back, and I think it is kind of the subliminal note that we will be hearing from Republicans all the way through.

INSKEEP: Ronald Brownstein is editorial director of National Journal. Thanks very much.